Self-cooled motor

ABSTRACT

A self-cooled motor includes a shaft; a rotor; a stator radially opposite the rotor; a housing supporting a bearing and the stator; and an impeller located axially below the rotor, and configured to rotate together with the shaft to generate an air current. The housing includes a base portion located axially above the rotor; two or more attachment portions radially outward of the rotor; and a cylindrical or substantially cylindrical cover portion configured to join the base portion and the attachment portions. A lower end of each attachment portion is located at an axial level lower than an axial level of the impeller. The base portion includes an air inlet. The cover portion includes an air outlet located between adjacent ones of the attachment portions, and configured to connect a space radially inside the housing and a space radially outside the housing with each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a self-cooled motor, and morespecifically to an improvement in a motor including a cooling bladeconfigured to rotate together with a shaft.

2. Description of the Related Art

A common microwave oven includes a stirring fan arranged to stir airinside a heating chamber. The stirring fan is arranged inside theheating chamber, and is driven by a stirring motor arranged outside theheating chamber. The stirring motor is attached to an outside of a wallsurface of the heating chamber with a shaft thereof projecting into theheating chamber. Thus, the stirring motor is in such an environment thatheat is easily transferred from the heating chamber to the stirringmotor, and therefore, an AC motor which is excellent in heat resistingproperty is used as the stirring motor.

The temperature of an interior of the heating chamber of the microwaveoven typically becomes 300° C. or higher. On the other hand, a commonmagnet has a heat-resistant temperature of about 150° C., and decreasesin coercive force when the temperature of the magnet exceeds about 130°C. Moreover, electronic components have heat-resistant temperaturesstill lower than the heat-resistant temperature of the magnet.Therefore, a brushless DC motor including a magnet and electroniccomponents cannot be used as the stirring motor, and instead, an ACmotor is typically used as the stirring motor.

With the AC motor, it is impossible to perform control of a rotationrate and a rotation direction more finely than with the brushless DCmotor. Therefore, known microwave ovens have a problem in that it isimpossible to finely control the rotation rate and the rotationdirection of the stirring fan to prevent uneven cooking and an uneventemperature distribution. In other words, the known microwave ovens havea problem in that it is difficult to improve functionality by employingthe stirring fan.

It is then conceivable to employ a brushless DC motor including acooling mechanism as the stirring motor to improve the functionality ofthe microwave oven. A variety of techniques have been proposed withrespect to the cooling mechanism of the brushless DC motor (see JP-A2008-154369, JP-A 2000-184644, JP-UM-A 62-178777, and JP-A 2000-050575).

However, the cooling mechanism disclosed in each of the aforementionedpatent documents is designed to discharge heat generated inside thebrushless DC motor to an outside, and is therefore unable tosufficiently cool the brushless DC motor when the brushless DC motor isarranged in the vicinity of an external heat source, such as the heatingchamber of the microwave oven.

For example, a motor described in each of the aforementioned patentdocuments includes a cooling mechanism which causes a blade arranged ona side opposite to a side where an attachment surface exists to rotateto send air into the motor from the side opposite to the side where theattachment surface exists. This cooling mechanism is unable tosufficiently reduce the likelihood that a high-temperature air in thevicinity of the attachment surface will flow into the motor. Moreover,because radiant heat from the attachment surface, which has a hightemperature, is not taken into consideration, this cooling mechanism isunable to exhibit sufficient cooling performance.

SUMMARY OF THE INVENTION

In view of the above circumstances, preferred embodiments of the presentinvention provide a self-cooled motor which is capable of being used ina high temperature environment, and, in particular, provide aself-cooled motor which is capable of being attached to an attachmentsurface having a high temperature, such as a wall surface of a hightemperature chamber of a microwave oven, and used.

A self-cooled motor according to a preferred embodiment of the presentinvention is configured to be attached to a wall surface of a hightemperature chamber through two or more attachment portions, andincludes a shaft extending into the high temperature chamber, theself-cooled motor including the shaft, the shaft being supported by abearing so as to be rotatable about a rotation axis extending in avertical direction; a rotor configured to rotate together with theshaft; a stator radially opposite the rotor; a housing configured tosupport the bearing and the stator; and an impeller located axiallybelow the rotor, and configured to rotate together with the shaft togenerate an air current traveling radially outward.

The housing includes a base portion located axially above the rotor; thetwo or more attachment portions, each attachment portion being locatedradially outward of the rotor; and a cylindrical or substantiallycylindrical cover portion configured to join the base portion and theattachment portions to each other. A lower end of each attachmentportion is located at an axial level lower than an axial level of theimpeller. The base portion includes an air inlet. The cover portionincludes an air outlet located between adjacent ones of the attachmentportions, and configured to connect a space radially inside the housingand a space radially outside the housing with each other.

When the above-described structure is adopted, rotation of the shaftcauses the impeller to rotate to generate the air current travelingradially outward, and air in a space between the wall surface of thehigh temperature chamber and the rotor holder is discharged through theair outlet. Accordingly, outside air is taken in through the air inletdefined in the base portion of the housing, and the outside air taken intravels in an axial direction inside the cover portion of the housingtoward the wall surface of the high temperature chamber. Accordingly, ahigh-temperature air in the vicinity of the wall surface of the hightemperature chamber is discharged radially along the wall surface of thehigh temperature chamber, and the outside air, which has a lowtemperature, is taken in through the base portion on a side opposite toa side where the wall surface of the high temperature chamber exists,such that a flow of air which passes axially downward inside the rotorholder is generated to cool an interior of the motor.

Preferred embodiments of the present invention provide a self-cooledmotor which is capable of being used in a high temperature environment.In particular, preferred embodiments of the present invention are ableto provide a self-cooled motor which is configured to be attached to anattachment surface having a high temperature, such as, for example, awall surface of a high temperature chamber of a microwave oven, andused.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary structure of a microwaveoven 500 including a motor 100 according to a first preferred embodimentof the present invention.

FIG. 2 is a cross-sectional view of the motor 100 according to the firstpreferred embodiment of the present invention.

FIG. 3 is an exploded perspective view of the motor 100 illustrated inFIG. 2, illustrating components of the motor 100 separated from oneanother in an axial direction.

FIG. 4 is a front view of the motor 100 illustrated in FIG. 2.

FIG. 5 is a left side view of the motor 100 illustrated in FIG. 2.

FIG. 6 is a plan view of the motor 100 illustrated in FIG. 2.

FIG. 7 is a bottom view of the motor 100 illustrated in FIG. 2.

FIG. 8 is a diagram illustrating relative positions of an impeller 6 anda rotor 2 according to the first preferred embodiment of the presentinvention.

FIG. 9 is a diagram explaining air currents generated by rotation of afirst comparative example of the impeller 6.

FIG. 10 is a diagram explaining the air currents generated by therotation of a second comparative example of the impeller 6.

FIG. 11 is a diagram explaining the air currents generated by therotation of a first example structure of the impeller 6 according to thefirst preferred embodiment of the present invention.

FIG. 12 is a diagram explaining the air currents generated by therotation of a second example structure of the impeller 6 according tothe first preferred embodiment of the present invention.

FIG. 13 is a diagram explaining an air current generated by rotation ofthe rotor 2.

FIG. 14 is a diagram illustrating exemplary flows of air inside themotor 100.

FIG. 15 is a diagram illustrating an exemplary structure of a motor 101according to a second preferred embodiment of the present invention.

FIG. 16 is a diagram illustrating an exemplary structure of a motor 102according to a third preferred embodiment of the present invention.

FIG. 17 is a diagram illustrating an exemplary structure of a motor 103according to a fourth preferred embodiment of the present invention.

FIG. 18 is a diagram illustrating an exemplary structure of a motor 104according to a fifth preferred embodiment of the present invention.

FIG. 19 is a diagram illustrating an exemplary structure of a motor 105according to a sixth preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. For the sake ofconvenience in description, a direction parallel or substantiallyparallel to a rotation axis J of a motor is herein referred to as avertical direction. However, this definition of the vertical directionshould not be construed to restrict in any way the orientation of amotor according to any preferred embodiment of the present inventionwhen in use. In addition, the direction parallel or substantiallyparallel to the rotation axis J of the motor is referred to simply bythe term “axial direction”, “axial”, or “axially”, radial directionscentered on the rotation axis J are referred to simply by the term“radial direction”, “radial”, or “radially”, and a circumferentialdirection about the rotation axis J is referred to simply by the term“circumferential direction”, “circumferential”, or “circumferentially”.

First Preferred Embodiment

FIG. 1 is a diagram illustrating an exemplary structure of a microwaveoven 500 including a motor 100 according to a first preferred embodimentof the present invention. The microwave oven 500 is a cooking apparatusincluding a high temperature chamber 501. The high temperature chamber501 preferably includes a heating chamber 502 and a stirring chamber 503divided by a wire net 504.

The heating chamber 502 is a space which accommodates, for example, foodas an object to be heated. Heating devices 510 are preferably arrangedabove and below the heating chamber 502. Each heating device 510 is adevice which heats the food inside the high temperature chamber 501. Aheater or a microwave generating device, such as for example amagnetron, is preferably used as each heating device 510. The stirringchamber 503 is a space in which a stirring fan 511 is arranged. Thestirring fan 511 is caused to rotate to stir air to change a temperaturedistribution inside the heating chamber 502. The stirring fan 511 isdriven by the motor 100, which is attached to a side wall of thestirring chamber 503. The wire net 504 is provided to prevent aninterference between the food inside the heating chamber 502 and thestirring fan 511, and for the safety of a user of the microwave oven500. The air is able to freely pass through the wire net 504.

The motor 100 is a driving source of the stirring fan 511, and ispreferably attached to an outer surface of a wall surface 505 of thehigh temperature chamber 501 with a shaft 1 of the motor 100 projectinginto the high temperature chamber 501. The wall surface 505 of the hightemperature chamber 501 will be hereinafter referred to as a “hightemperature chamber wall surface” 505. More specifically, the motor 100is arranged outside the high temperature chamber 501, the shaft 1extends toward the high temperature chamber 501, passing through athrough hole 506 defined in the high temperature chamber wall surface505, and the stirring fan 511 is attached to a portion of the shaft 1near a top thereof, the portion being located inside the stirringchamber 503.

FIG. 2 is a cross-sectional view of the motor 100 according to the firstpreferred embodiment taken along a plane including a rotation axis J.FIG. 3 is an exploded perspective view of the motor 100, illustratingcomponents of the motor 100 separated from one another in an axialdirection.

The motor 100 is a self-cooled motor including an impeller 6 configuredto rotate together with the shaft 1. The impeller 6 is located on a sidewhere the high temperature chamber wall surface 505, which defines andserves as an attachment surface, exists. Once the shaft 1 startsrotating, the impeller 6 discharges a high-temperature air in thevicinity of the attachment surface radially outward. This contributes toreducing an increase in the temperature inside the motor 100. Inaddition, the impeller 6 is preferably arranged to block radiant heatfrom the attachment surface, and this contributes to reducing theincrease in the temperature inside the motor 100. Further, the motor 100includes a channel passing therethrough in the axial direction, and anair current passing therethrough is generated. Specifically, outside airis taken into the motor 100 from a side opposite to the side where theattachment surface exists, passes axially within the motor 100 towardthe attachment surface, and is discharged radially outward in thevicinity of the attachment surface. As a result, air within the motor100 is replaced with the outside air, which has a lower temperature, andan interior of the motor 100 is cooled.

The motor 100 is preferably an outer-rotor motor in which a rotor 2 isfixed to the shaft 1, and in which a stator 3 is radially inside andopposite the rotor 2 with a gap intervening therebetween. The motor 100includes a stationary portion fixed to the high temperature chamber wallsurface 505 of the microwave oven 500, and a rotating portion rotatablysupported by the stationary portion. The rotating portion preferablyincludes the shaft 1, the rotor 2, and the impeller 6. Meanwhile, thestationary portion preferably includes the stator 3, a circuit board 4,a housing 5, two bearings 10, and a bearing holding portion 11.Hereinafter, each of these components will be described in detail.

The shaft 1 is a columnar or substantially columnar member extending inthe axial direction (i.e., the vertical direction). The shaft 1 isconfigured to rotate about the rotation axis J while being supported bythe two bearings 10. A lower end portion of the shaft 1 projectsdownward below the housing 5. This projecting lower end portion isconfigured to pass through the through hole 506 of the high temperaturechamber wall surface 505. The stirring fan 511 is attached to theprojecting lower end portion at a position closer to a top of theprojecting lower end portion than to the high temperature chamber wallsurface 505.

The rotor 2 is a member configured to rotate together with the shaft 1,and preferably includes a rotor holder 20 and a rotor magnet 21. Therotor holder 20 has a bottom and is cylindrical in shape, and includes acylindrical portion 22 and a bottom plate portion 23. The rotor holder20 has an upper opening 25 axially above. The cylindrical portion 22 iscylindrical or substantially cylindrical, and is located radiallyoutside the stator 3. The bottom plate portion 23 is in the shape of aplate, and extends radially inward from a lower end of the cylindricalportion 22. The bottom plate portion 23 is located below the stator 3,and is fixed to the shaft 1. The rotor magnet 21 is preferably apermanent magnet, and is fixed to an inner circumferential surface ofthe cylindrical portion 22 of the rotor holder 20.

The bottom plate portion 23 of the rotor holder 20 preferably includes athrough hole 2 j for the shaft 1, and rotor vent holes 24 locatedradially outside the shaft 1. Each rotor vent hole 24 is, for example, aradially elongated opening. The rotor vent holes 24, numbering two ormore, are located in a circumferential direction. According to thepresent preferred embodiment, the rotor vent holes 24 are arranged atregular intervals in the circumferential direction. Provision of theserotor vent holes 24 enables air to pass through the bottom plate portion23 in the axial direction. This enables air inside the rotor holder 20to be discharged toward the impeller 6.

In particular, when at least a portion of each rotor vent hole 24 islocated radially inward of an inner circumferential surface of the rotormagnet 21, air flowing in the axial direction inside the rotor holder 20is able to smoothly pass through the rotor vent hole 24. In addition,when at least a portion of each rotor vent hole 24 is located radiallyoutward of an outer circumferential surface of a core back 30 b of thestator 3, the air flowing in the axial direction inside the rotor holder20 is able to smoothly pass through the rotor vent hole 24.

The stator 3 is an armature of the motor 100, preferably includes astator core 30 and coils 31, and is radially inside the rotor 2. Thestator 3 is annular or substantially annular, and an outercircumferential surface of the stator 3 is located radially opposite therotor magnet 21 with a gap intervening therebetween.

The stator core 30 is preferably, for example, defined by laminatedsteel sheets, i.e., magnetic steel sheets, such as silicon steel sheets,placed one upon another in the axial direction. The stator core 30includes the core back 30 b, which is annular, and a plurality of teeth30 t arranged to project radially outward from the outer circumferentialsurface of the core back 30 b. A winding is wound around each of theteeth 30 t to define the coils 31. Once drive currents are supplied tothe coils 31, radial magnetic flux is generated around each of the teeth30 t, each of which is a magnetic core. A circumferential torque is thusproduced between the teeth 30 t and the rotor magnet 21, so that theshaft 1 is caused to rotate about the rotation axis J.

The circuit board 4 is a board on which an electronic circuit (notshown) designed to supply the electric drive current to the coils 31 ismounted, and is a circular or substantially circular plate-shaped body.Electronic components, including a semiconductor device, are mounted onthe circuit board 4. In addition, a magnetic sensor and a connector maypreferably be mounted on the circuit board 4. The circuit board 4preferably has an outside diameter greater than the outside diameter ofthe rotor holder 20, is located axially above the rotor 2, and isarranged opposite to the upper opening 25 of the rotor holder 20.

The circuit board 4 preferably includes a through hole 4 j for the shaft1, and a board vent hole 4 h located radially outside the shaft 1. Theboard vent hole 4 h is, for example, a circumferentially elongatedopening. Provision of the board vent hole 4 h as described above enablesair to pass through the circuit board 4 in the axial direction.

The board vent hole 4 h is preferably located radially inward of thecylindrical portion 22 of the rotor holder 20, and is arranged oppositeto the upper opening 25 of the rotor 2. Accordingly, good ventilation issecured above the rotor 2. That is, when at least a portion of the boardvent hole 4 h overlaps with the upper opening 25 of the rotor 2 whenviewed in the axial direction, air is able to smoothly pass in the axialdirection between an inside of the rotor 2 and a space above the circuitboard 4.

Moreover, only a slight gap is defined between the circuit board 4 andan upper end of the cylindrical portion 22 of the rotor holder 20, andonly a limited amount of air is permitted to pass radially through thisgap. For example, the circuit board 4 is arranged opposite to the upperend of the cylindrical portion 22 of the rotor holder 20 while beingspaced therefrom by a distance smaller than the radial thickness of therotor magnet 21. The gap between the circuit board 4 and the upper endof the cylindrical portion 22 of the rotor holder 20 preferably has avery small width to limit a radial flow of air through the gap and topermit an axial flow of air through the board vent hole 4 h inpreference to the radial flow of air through the gap.

Each bearing 10 is a member configured to rotatably support the shaft 1.A ball bearing, for example, is preferably used as each bearing 10. Eachof the two bearings 10 is press fitted and fixed to the shaft 1, and isloose fitted to the bearing holding portion 11. These bearings 10 areaccommodated inside the bearing holding portion 11 with a spacer 12,which is fixed inside the bearing holding portion 11, held therebetween.

The bearing holding portion 11 is preferably a bracket whichaccommodates the bearings 10, and is obtained by subjecting a metalsheet, such as, for example, a galvanized steel sheet, to press working.The bearing holding portion 11 includes a cylindrical portion 11 c pressfitted and fixed to an inner circumferential surface of the stator 3,and a flange portion 11 f extending radially outward from an upper endof the cylindrical portion 11 c. The spacer 12, which is cylindrical orsubstantially cylindrical, is accommodated inside the cylindricalportion 11 c. The two bearings 10 are supported by the bearing holdingportion 11 as a result of the spacer 12 being press fitted inside thebearing holding portion 11. The flange portion 11 f is arranged above abase portion 50 of the housing 5. A lower surface of the flange portion11 f includes boss portions 11 b each of which extends axially downward,and each boss portion 11 b is located in a through hole 50 b defined inthe base portion 50 of the housing 5. The flange portion 11 f isaccordingly supported so as to be incapable of turning relative to thehousing 5.

The housing 5 preferably includes the base portion 50, which is locatedaxially above the circuit board 4, a cover portion 51 located radiallyoutside the rotor 2, and attachment portions 52 each of which isattached to the high temperature chamber wall surface 505, which ispreferably located axially below the impeller 6. The housing 5 is in orsubstantially in the shape of a covered cylinder with a downwardopening, and is easily produced by subjecting a metal sheet, such as,for example, an aluminum sheet, to press working, for example.

The base portion 50 is in or substantially in the shape of a circularplate, and extends radially inward from an upper end of the coverportion 51. The base portion 50 is configured to support the stator 3and the two bearings 10 through the bearing holding portion 11. The baseportion 50 preferably includes supporting projections 50 p each of whichprojects axially downward. Each supporting projection 50 p is configuredto support the circuit board 4. An upper surface of the circuit board 4is arranged opposite to a lower surface of the base portion 50 with agap intervening therebetween. Each supporting projection 50 p ispreferably defined by, for example, subjecting the base portion 50 to alancing process. In addition, the base portion 50 includes a throughhole 5 j for the bearing holding portion 11, and air inlets 54 locatedradially outside the bearing holding portion 11. Provision of the airinlets 54 in the base portion 50 enables the outside air to enter intothe housing 5.

The cover portion 51 is a cylindrical or substantially cylindricalportion located radially outward of both the rotor 2 and the circuitboard 4. An inner circumferential surface of the cover portion 51 isarranged opposite to an outer circumferential surface of the cylindricalportion 22 of the rotor holder 20 with a gap intervening therebetween.Similarly, the inner circumferential surface of the cover portion 51 isarranged opposite to an outer circumferential surface of the circuitboard 4 with a gap intervening therebetween. The upper end of the coverportion 51 is joined to the base portion 50. In addition, the attachmentportions 52, which number two or more, are arranged at a lower end ofthe cover portion 51. That is, the cover portion 51 is configured tojoin each of the two or more attachment portions 52 to the base portion50. Moreover, two or more main air outlets 55 are preferably located inthe circumferential direction below the cover portion 51. Furthermore,the cover portion 51 includes a secondary air outlet 57 defined by anopening passing therethrough in a radial direction at onecircumferential position. Note that, although the base portion 50 andeach attachment portion 52 are joined to each other through thecylindrical or substantially cylindrical cover portion 51 according tothe present preferred embodiment, the cover portion 51 may be in anyshape other than the cylindrical shape. Also note that the housing 5 maynot necessarily include the cover portion 51, and that each attachmentportion 51 may alternatively be extending axially downward from an outerperiphery portion of the base portion 50.

Each attachment portion 52 is a member fixed to the high temperaturechamber wall surface 505, which defines and serves as the attachmentsurface, and is a plate-shaped portion extending radially outward fromthe lower end of the cover portion 51. For example, the two or moreattachment portions 52 are arranged at regular intervals in thecircumferential direction. Each attachment portion 52 includes a hole 52h defined therein, and a fastener, such as a screw, may be used to fixthe attachment portion 52 to the high temperature chamber wall surface505. That is, the motor 100 is attached to the high temperature chamberwall surface 505 with a lower surface of each attachment portion 52being in contact with the outer surface of the high temperature chamberwall surface 505.

The impeller 6 preferably includes a rotating plate 61 and two or moreblades 60 arranged thereon, is located axially below the rotor 2, andpreferably has an outside diameter smaller than the inside diameter ofthe cover portion 51 of the housing 5. The impeller 6 is arranged at anaxial level higher than an axial level of a lower end of each attachmentportion 52. The impeller 6 is configured to rotate together with theshaft 1 to generate an air current traveling radially outward betweenthe rotor 2 and the high temperature chamber wall surface 505. Theimpeller 6 is easily produced by subjecting a metal sheet to a stampingprocess or the like, for example.

The rotating plate 61 is preferably in or substantially in the shape ofa circular plate, and is fixed to the shaft 1. The two or more blades 60are arranged on a lower surface of the rotating plate 61, and the blades60 are caused to rotate together with the shaft 1. In addition, therotating plate 61 preferably has an outside diameter greater than theoutside diameter of the rotor holder 20, and is located axially oppositethe bottom plate portion 23 of the rotor holder 20 with a gapintervening therebetween. Accordingly, the rotating plate 61 is able toblock the radiant heat from the high temperature chamber wall surface505 to reduce an increase in the temperature of each of the rotor 2 andthe circuit board 4. In addition, the rotating plate 61 includes athrough hole 6 j for the shaft 1, and openings 63 permitting ventilationlocated radially outside the shaft 1. These openings 63 enable air topass through the rotating plate 61 in the axial direction.

Each blade 60 is preferably a rectangular or substantially rectangularplate-shaped body projecting axially downward from the rotating plate61, and is defined by, for example, subjecting the rotating plate 61 toa lancing process. Holes defined in the rotating plate 61 as a result ofthis process preferably are used as the openings 63. The two or moreblades 60 are arranged at regular or substantially regular intervals inthe circumferential direction on the rotating plate of the impeller 6.Each blade 60 is a plate-shaped body located in parallel orsubstantially in parallel with the rotation axis J, and extends in aradial direction. Therefore, once the shaft 1 starts rotating, theblades 60 are caused to rotate about the rotation axis J to generate theair current traveling radially outward through a centrifugal force.Moreover, this air current always flows radially outward regardless of arotation direction of the shaft 1. Further, generation of the aircurrent traveling radially outward produces a negative pressure radiallyinside each blade 60.

The rotating plate 61 further includes two or more ribs 62. Each rib 62is a projection portion defined by bending a portion of the rotatingplate 61 to increase the strength of the rotating plate 61. Each rib 62is defined by subjecting the rotating plate 61 to press working, forexample. The two or more ribs 62 are arranged at regular intervals inthe circumferential direction in the rotating plate 61. In addition,each rib 62 is arranged between adjacent ones of the blades 60, projectsin the same direction as a direction in which each blade 60 projectsfrom the rotating plate 61, and is in an elongated shape extending in aradial direction. Accordingly, the ribs 62 promote the generation of theair current traveling radially outward caused by the blades 60.

FIGS. 4, 5, 6, 7, and 8 are external views illustrating an exemplarystructure of the motor 100 according to the present preferredembodiment. FIG. 4 is a front view of the motor 100, illustrating thesecondary air outlet 57 as viewed from radially outside. FIG. 5 is aleft side view of the motor 100, illustrating the motor 100 as viewedfrom the left-hand side in FIG. 4. FIG. 6 is a plan view of the motor100, illustrating the motor 100 as viewed from above in the axialdirection. FIG. 7 is a bottom view of the motor 100, illustrating themotor 100 as viewed from below in the axial direction. FIG. 8illustrates relative positions of the impeller 6 and the rotor 2 whenviewed from below in the axial direction. Note that the cross-sectionalview of FIG. 2 is taken along line A-A illustrated in FIGS. 4 and 6.

Referring to FIG. 6, the air inlets 54 will now be described below. Eachair inlet 54 is preferably an opening defined in the base portion 50 ofthe housing 5. For example, two or more openings each of which istrapezoidal or substantially trapezoidal are defined as the air inlets54. Moreover, holes defined as a result of the lancing process of thesupporting projections 50 p may also be used as the air inlets 54.Provision of these air inlets 54 enables air to pass through the baseportion 50 in the axial direction, permitting the outside air axiallyabove the housing 5 to be taken into the housing 5.

In particular, since the gap is defined between the base portion 50 andthe circuit board 4, easy passage of air between each air inlet 54 andthe board vent hole 4 h is ensured. Moreover, when any of the air inlets54 is arranged opposite to the board vent hole 4 h, easier passage ofair between the air inlet 54 and the board vent hole 4 h is ensured. Forexample, when the air inlets 54 are defined in the base portion 50 suchthat at least a portion of any air inlet 54 axially overlaps with theboard vent hole 4 h, easy passage of air between the air inlet 54 andthe board vent hole 4 h is ensured. Note that, even in the case where nogap is defined between the base portion 50 and the circuit board 4, easyaxial passage of air is ensured when any air inlet 54 is arrangedopposite to the board vent hole 4 h.

Referring to FIGS. 4 and 5, the main air outlets 55 will now bedescribed below. Each main air outlet 55 is preferably an openingdefined below the cover portion 51 of the housing 5. Each main airoutlet 55 is a gap defined between the high temperature chamber wallsurface 505 and an outlet edge portion 56 when the motor 100 has beenattached to the high temperature chamber wall surface 505. The outletedge portion 56 is a lower edge portion of the cover portion 51, and isdefined at a position circumferentially between adjacent ones of theattachment portions 52. That is, each main air outlet 55 is a gapdefined between the high temperature chamber wall surface 505 and thecover portion 51 of the housing 5, is arranged between circumferentiallyadjacent ones of the attachment portion 52, and is configured to connecta space radially inside the housing 5 and a space radially outside thehousing 5 with each other. Provision of the main air outlets 55 asdescribed above enables air inside the housing 5 to be discharged to anoutside through each main air outlet 55 through the air currenttraveling radially outward and caused by the blades 60, to cool aninterior of the housing 5.

Each outlet edge portion 56 is preferably defined, for example, as a cutportion at the lower end of the cover portion 51. In this case, sinceeach outlet edge portion 56 is arranged at an axial level higher than anaxial level of the lower end of each attachment portion 52, the gap ispreferably defined between the outlet edge portion 56 and the hightemperature chamber wall surface 505 when the attachment portions 52 arearranged to be in contact with the high temperature chamber wall surface505, and this gap defines and serves as the main air outlet 55.

Each main air outlet 55 preferably has an elongated shape extending inthe circumferential direction, to achieve efficient discharge of the airthrough the main air outlet 55. In particular, each main air outlet 55preferably has a greater circumferential dimension, specifically, agreater circumferential dimension than the circumferential dimension ofeach attachment portion 52. In addition, two or more of such outlet edgeportions 56 are preferably located in a circumferential direction of thecover portion 51 to increase the total area of the main air outlets 55.Further, an outer edge of each blade 60 is preferably opposed to eachoutlet edge portion 56. For example, when each outlet edge portion 56 islocated at an axial level higher than an axial level of a lower end ofeach blade 60, at least a portion of the outer edge of each blade 60will be opposed to each main air outlet 55 to facilitate the dischargeof the air.

Furthermore, each outlet edge portion 56 preferably includes a collarportion projecting radially outward, in order to more efficiently coolthe motor 100. Provision of the collar portion causes a dischargedhigh-temperature air to travel farther away from the motor 100 to reducethe likelihood that the discharged high-temperature air will return intothe motor 100 through any of the air inlets 54 and the secondary airoutlet 57 of the housing 5 or the like.

Referring to FIGS. 4 and 6, the secondary air outlet 57 of the housing 5will now be described below. The secondary air outlet 57 is preferablyan opening defined as a result of cutting off a circumferential portionof the housing 5. The secondary air outlet 57, which has a specifiedcircumferential dimension and extend in the axial direction, is definedin the cover portion 51 of the housing 5. Provision of the secondary airoutlet 57 as described above enables air in a space radially outside therotor 2 and inside the housing 5 to be discharged to the outside throughrotation of the rotor 2 to cool the interior of the housing 5.

A lower end of the secondary air outlet 57 reaches one of the outletedge portions 56 of the cover portion 51, and the secondary air outlet57 is joined to one of the main air outlets 55. That is, the secondaryair outlet 57 is arranged circumferentially between adjacent ones of theattachment portions 52. In addition, an upper end of the secondary airoutlet 57 reaches the upper end of the cover portion 51, then intrudesinto the base portion 50, and is joined to one of the air inlets 54.

In order to efficiently discharge air through the secondary air outlet57, the secondary air outlet 57 preferably has a circumferential extentof less than 180 degrees. That is, the upper end of the cover portion 51is preferably joined to the base portion 50 over an angular range of 180or more degrees. Moreover, it is preferable that only one secondary airoutlet 57, provided at one circumferential position, should be defined.

Referring to FIG. 7, the blades 60 of the impeller 6 will now bedescribed below. Each blade 60 is configured to generate the air currenttraveling radially outward through the centrifugal force. Therefore, themore radially outward an outer circumferential end of the blade 60 is,the more efficiently the blade 60 is able to generate the air current.Therefore, the outer circumferential end of the blade 60 is preferablylocated more radially outward. For example, the outer circumferentialend of the blade 60 is preferably located radially outward of the outercircumferential surface of the cylindrical portion 22 of the rotorholder 20. In FIG. 7, the outer edge of each blade preferably coincideswith an outer circumferential edge of the rotating plate 61.

Moreover, a negative pressure is produced radially inside the outercircumferential end of each blade 60. Accordingly, the outercircumferential end of each blade 60 needs to be located at leastradially outward of each rotor vent hole 24 to allow the negativepressure to cause an axial passage of air within the rotor 2.

Referring to FIGS. 7 and 8, the openings 63 of the impeller 6 will nowbe described below. Each opening 63 is an opening used for ventilationand defined in the rotating plate 61 of the impeller 6. The openings 63,which preferably number two or more, for example, are arranged atregular or substantially regular intervals in the circumferentialdirection. Provision of the openings 63 as described above enables axialpassage of the air through the rotating plate 61.

Each of the openings 63 preferably has an elongated shape extending in aradial direction, and one end of the opening 63 is located radiallyinward of a corresponding one of the blades 60. For example, in the casewhere each blade 60 is defined by the lancing process, a cut holedefined as a result of defining the blade 60 is arrangedcircumferentially adjacent to the blade 60. In this case, the opening 63is preferably defined by expanding this cut hole farther radiallyinward. Provision of such openings 63 contributes to effectivelygenerating an air current passing axially downward through the rotatingplate 61 when the negative pressure has been produced as a result of therotation of the blades 60.

When the openings 63 are defined such that the openings 63 overlap withthe rotor vent holes 24 when viewed in the axial direction, easy passageof air between the rotor vent holes 24 and the openings 63 is ensured.Meanwhile, when the openings 63 are defined such that the openings 63 donot overlap with the rotor vent holes 24 when viewed in the axialdirection, the radiant heat from the high temperature chamber wallsurface 505 is effectively blocked. Therefore, when the easy passage ofair should take precedence over the blocking of the radiant heat, theopenings 63 are defined such that the openings 63 overlap with the rotorvent holes 24 when viewed in the axial direction, whereas when theblocking of the radiant heat should take precedence over the easypassage of air, the openings 63 are defined such that the openings 63 donot overlap with the rotor vent holes 24 when viewed in the axialdirection.

In FIGS. 7 and 8, the openings 63 and the rotor vent holes 24 areconfigured to adjoin or substantially adjoin each other whileoverlapping with each other when viewed in the axial direction, so thatthe easy passage of air and the blocking of the radiant heat arebalanced. More specifically, the openings 63 are located more radiallyoutward than the rotor vent holes 24, and the openings 63 and the rotorvent holes 24 are located to radially adjoin or substantially adjoineach other. While the openings 63 and the rotor vent holes 24 slightlyoverlap with each other, a half or more of the area of each rotor venthole is covered with the rotating plate 61, and therefore, a sufficientblocking effect is obtained.

FIGS. 9, 10, 11, 12, and 13 are diagrams explaining a cooling principleof the motor 100 according to the present preferred embodiment. FIGS. 9to 12 are diagrams explaining air currents F1, F2, and F3 generated byrotation of the impeller 6, and each illustrates a structure defined bythe shaft 1, the rotor holder 20, and the impeller 6 in a simplifiedform. Structures according to comparative examples are illustrated inFIGS. 9 and 10, whereas example structures according to the presentpreferred embodiment are illustrated in FIGS. 11 and 12.

Each of the rotor holder 20 and the impeller 6 is fixed to the shaft 1,and is configured to rotate about the rotation axis J. The rotor holder20 preferably includes the rotor vent holes 24 defined in the bottomplate portion 23 thereof. The impeller 6 is located below the bottomplate portion 23. The impeller 6 preferably includes the rotating plate61 and the two or more blades 60 arranged thereon, and each blade 60extends in a radial direction and in parallel with the rotation axis J.In addition, the openings 63 (not shown) are defined in the rotatingplate 61.

FIG. 9 illustrates, as a first comparative example, a structure in whichthe rotor vent holes 24 are located radially outward of the blades 60,and in which there is an open space axially below the impeller 6. Oncethe impeller 6 starts rotating, air between every adjacent ones of theblades 60 also rotates, and the air current F1, which travels radiallyoutward, is generated by a resulting centrifugal force. As a result, anatmospheric pressure is reduced radially inside the blades 60, and anegative pressure is produced in the vicinity of a center of theimpeller 6.

The negative pressure would cause air axially above or below theimpeller 6 to be drawn toward the vicinity of the center of the impeller6. However, the bottom plate portion 23 of the rotor holder 20 blocks aspace above the impeller 6. Therefore, air flows into the vicinity ofthe center of the impeller 6 from the open space axially below theimpeller 6. That is, the air current F2, which flows axially upward inthe vicinity of the shaft 1 toward the impeller 6, is generated. Sincethe rotor vent holes 24 are located radially outward of the blades 60, aportion of the air current F1, which travels radially outward, flowsinto the rotor holder 20 through each rotor vent hole 24.

FIG. 10 illustrates, as a second comparative example, a structure inwhich the rotor vent holes 24 are located radially inward of the blades60, and in which there is an open space axially below the impeller 6.Once the impeller 6 starts rotating, the air current F1, which travelsradially outward, is generated, and a negative pressure is produced inthe vicinity of the center of the impeller 6, as in the case of FIG. 9.

In this case, air is drawn toward the vicinity of the center of theimpeller 6 from both axially above and axially below. However, the airis more easily drawn from the open space axially below than from axiallyabove through the rotor vent holes 24. Therefore, as in the case of FIG.9, the air current F2, which flows axially upward in the vicinity of theshaft 1 toward the impeller 6, is generated. At this time, a portion ofthe air current F2 flows into the rotor holder 20 through each rotorvent hole 24.

FIG. 11 illustrates, as a first example structure according to thepresent preferred embodiment, a structure in which the rotor vent holes24 are located radially inward of the blades 60, and in which a spaceaxially below the blades 60 is closed with the high temperature chamberwall surface 505. Once the impeller 6 starts rotating, the air currentF1, which travels radially outward, is generated, and a negativepressure is produced in the vicinity of the center of the impeller 6, asin the cases of FIGS. 9 and 10.

A portion of the shaft 1 is accommodated in the through hole 506 of thehigh temperature chamber wall surface 505, and no gap is defined betweenan outer circumferential surface of the shaft 1 and an innercircumferential surface of the through hole 506. That is, the spaceaxially below the impeller 6 is closed with the high temperature chamberwall surface 505. Therefore, the negative pressure produced in thevicinity of the center of the impeller 6 generates the air current F2,which flows axially downward through each rotor vent hole 24.

That is, once the air inside the rotor holder 20 travels toward the hightemperature chamber wall surface 505 and reaches the impeller 6, the airturns radially outward and is discharged out of the housing 5. Since thetemperature of the high temperature chamber wall surface 505 becomeshigh, air having a high temperature exists in the vicinity of the hightemperature chamber wall surface 505. However, generation of the aircurrents F1 and F2 as described above reduces the likelihood that theair in the vicinity of the high temperature chamber wall surface 505will flow into the rotor holder 20. In addition, new outside air istaken into the rotor holder 20 from axially above. Further, the airhaving the high temperature and existing in the vicinity of the hightemperature chamber wall surface 505 is discharged out of the housing 5together with the air inside the rotor 2.

FIG. 12 illustrates, as a second example structure according to thepresent preferred embodiment, a structure in which the impeller 6 has anoutside diameter greater than the outside diameter of the rotor holder20. Because relative positions of the rotor vent holes 24, the blades60, and the high temperature chamber wall surface 505 are the same as inthe case of FIG. 11, the air currents F1 and F2 generated are the sameas those generated in the case of FIG. 11.

Since the outside diameter of the rotating plate 61 is greater than theoutside diameter of the rotor holder 20, a negative pressure produced bythe impeller 6 generates the air current F3, which flows axiallydownward, radially outside the rotor holder 20. The negative pressure isproduced radially inside the outer circumferential ends of the blades60. Therefore, when the outer circumferential ends of the blades 60 arelocated radially outward of an outer circumferential surface of therotor holder 20, the air current F3, which flows axially downward, isgenerated radially outside the rotor holder 20. In addition, when thereis a gap between the rotating plate 61 of the impeller 6 and the bottomplate portion 23 of the rotor holder 20, the air current F3 flowsradially inward in this gap and joins the air current F2.

However, the air amount of the air current F3 is smaller than that ofthe air current F2, and the velocity of the air current F3 is lower thanthat of the air current F2. The negative pressure is produced radiallyinside the outer circumferential ends of the blades 60, and increaseswith decreasing distance from a center of rotation of the blades 60. Inaddition, a channel through which the air current F3 travels radiallyinward in a gap between the rotor holder 20 and the rotating plate 61 iscomplicated, and air is unable to flow smoothly in this channel.Therefore, the air current F3, which flows outside the rotor holder 20,is generated as a weaker flow of air than the air current F2, whichflows inside the rotor holder 20.

FIG. 13 is a diagram explaining an air current F4 generated by therotation of the rotor 2, and illustrates a cross section taken alongline B-B in FIG. 4 in a simplified form. The rotor holder 20 is opposedto the housing 5 with a radial gap intervening therebetween, and acylindrical space is defined radially outside the rotor 2. Morespecifically, the cylindrical space is located axially between thecircuit board 4 and the impeller 6 and radially between the cylindricalportion of the rotor holder 20 and the cover portion 51 of the housing5.

Air in the cylindrical space rotates together with the rotor 2.Therefore, when the secondary air outlet 57 is defined in the coverportion 51 of the housing 5, a portion of the rotating air is dischargedout of the housing 5 through the secondary air outlet 57 through acentrifugal force. Thus, the air radially outside the rotor holder 20 iscooled.

FIG. 14 is a diagram illustrating exemplary flows of air inside themotor 100, and illustrates air currents Fa1 and Fa2 each of which flowsin the axial direction inside the motor 100, and an air current Fb whichflows in the circumferential direction inside the motor 100.

First, the air current Fa1, which flows in the axial direction, will nowbe described below. The air current Fa1 is a flow of outside air whichis taken in from above the motor 100, then travels axially downwardthrough an interior of the rotor 2, and is then discharged radiallyoutward along the high temperature chamber wall surface 505. The aircurrent Fa1 is caused by the rotation of the impeller 6.

The outside air is taken into the housing 5 through each air inlet 54 ofthe base portion 50. Since the base portion is located on an oppositeside with respect to the high temperature chamber wall surface 505,outside air which is away from the high temperature chamber wall surface505 and which has a low temperature is taken into the motor 100 byintroducing the outside air through each of the air inlets 54 defined inthe base portion 50. The air taken into the housing 5 passes through theboard vent hole 4 h of the circuit board 4 to enter into the rotorholder 20 through the upper opening 25, and travels in the axialdirection inside the rotor holder 20.

Inside the rotor holder 20, the air flows axially downward between therotor magnet 21 and the core back 30 b. The teeth 30 t, which number twoor more, are arranged between the rotor magnet 21 and the core back 30b, and the air current Fa1 passes between adjacent ones of the teeth 30t. Then, the air reaches the bottom plate portion 23 of the rotor holder20, and then travels out of the rotor holder 20 through each rotor venthole 24.

After traveling out of the rotor holder 20, the air passes axiallydownward through each opening 63 of the impeller 6, and then turnsradially outward and is discharged out of the motor 100 through eachmain air outlet 55 of the housing 5. Because the air travels along thehigh temperature chamber wall surface 505 at this time, the air in thevicinity of the high temperature chamber wall surface 505 and having ahigh temperature is also discharged through each main air outlet 55.

An axial flow of air from each air inlet 54 of the base portion 50 toeach opening 63 of the impeller 6 is generated when a channel inside themotor 100 and passing through the motor 100 in the axial direction issecured, and the impeller 6 has produced a negative pressure at anaxially lower end of the channel. That is, inside the motor 100, achannel is secured for air entering through each air inlet 54, passingthrough the rotor holder 20 in the axial direction inside the rotorholder 20, and passing through each opening 63 of the impeller 6. Inaddition, the rotation of the blades 60 produces a negative pressureradially inside the outer circumferential end of each blade 60. Thus,the negative pressure causes an air current to flow axially downwardthrough the above channel.

A flow of air traveling radially outward is generated by the rotation ofthe impeller 6. The air which has passed inside and through the motor100 is discharged through each main air outlet 55 as the air is causedto flow radially outward along the high temperature chamber wall surface505. Moreover, since the air in the vicinity of the high temperaturechamber wall surface 505 and having a high temperature is alsodischarged radially outward, the likelihood that the temperature of eachof the rotor 2, the circuit board 4, and so on will increase because ofthe air having a high temperature traveling axially upward is reduced.

Next, the air current Fa2, which flows in the axial direction, will nowbe described below. The air current Fa2 is a flow of outside air whichis taken in from above the motor 100, then travels axially downwardradially outside the rotor 2, and is then discharged radially outwardalong the high temperature chamber wall surface 505. The air current Fa2is caused by the rotation of the impeller 6.

A gap is preferably defined between the upper surface of the circuitboard 4 and the base portion 50. In addition, another gap is preferablydefined between an outer circumferential end of the circuit board 4 andthe cover portion 51. Accordingly, outside air which has been taken intothe housing 5 through any air inlet 54 of the base portion 50 passesthrough these gaps to enter into a space between the rotor holder 20 andthe cover portion 51, and travels in the axial direction outside therotor holder 20. Then, after reaching the impeller 6, the air passesaxially downward through each opening of the impeller 6, and then turnsradially outward and is discharged out of the motor 100 through eachmain air outlet 55 of the housing 5. That is, the air joins the aircurrent Fa1. In the above-described manner, the outside air, which has alow temperature, is taken into the motor 100 and is caused to pass inthe axial direction radially outside the rotor 2, such that the interiorof the motor 100 is cooled.

Next, the air current Fb, which flows in the circumferential direction,will now be described below. The air current Fb is a flow of air whichtravels in the circumferential direction radially outside the rotor 2,and which is discharged radially through a side surface of the housing5. The air current Fb is caused by the rotation of the rotor 2. As isapparent from the above explanation of the air current Fa2, air suppliedto the space radially outside the cylindrical portion 22 of the rotorholder 20 is the outside air taken in from above the motor 100.

Once the rotor 2 starts rotating, the air in a gap between thecylindrical portion 22 of the rotor holder 20 and the cover portion 51of the housing 5 is caused to travel in the circumferential direction.That is, the air is caused to rotate about the rotation axis J. Then, aportion of the air which has reached the secondary air outlet 57 isdischarged out of the housing 5 through the secondary air outlet 57through the centrifugal force.

The structure and operation of the motor 100 according to the presentpreferred embodiment have been described above. Hereinafter, beneficialeffects obtained by the structure or the operation described above willbe described in detail.

The motor 100 according to the present preferred embodiment preferablyis an outer-rotor motor attached to the high temperature chamber wallsurface 505 when used. In the motor 100, the blades 60, which areconfigured to rotate together with the shaft 1 to generate the aircurrent traveling radially outward, are located on a side of the rotor 2closer to the high temperature chamber wall surface 505. In addition,the housing 5 includes the outlet edge portions 56, each of whichdefines a corresponding one of the main air outlets 55 together with thehigh temperature chamber wall surface 505. Further, the bottom plateportion 23 of the rotor holder 20, which is located on the side closerto the high temperature chamber wall surface 505, includes the rotorvent holes 24.

Adoption of the above arrangements makes it possible to effectively coolthe interior of the motor 100. Once the shaft 1 starts rotating, air ina space between the rotor 2 and the high temperature chamber wallsurface 505 is discharged through each main air outlet 55, and a spaceradially inside the outer circumferential ends of the blades 60 comesunder negative pressure. Accordingly, the air inside the rotor holder 20is caused to travel toward the space under negative pressure througheach of the rotor vent holes 24 defined in the bottom plate portion 23of the rotor holder 20. That is, the air inside the rotor holder 20passes through each rotor vent hole 24 toward the high temperaturechamber wall surface 505, and is discharged radially outward througheach main air outlet 55 together with the air in the vicinity of thehigh temperature chamber wall surface 505 and having a high temperature.Thus, the air in the vicinity of the high temperature chamber wallsurface 505 and having a high temperature is prevented from enteringinto the rotor holder 20, and an axially downward flow of air isgenerated inside the rotor holder 20 to cool the interior of the motor100.

In the motor 100 according to the present preferred embodiment, theouter circumferential ends of the blades 60 are located radially outwardof the rotor vent holes 24. Adoption of the above arrangement makes itpossible to use the negative pressure produced radially inside the outercircumferential ends of the blades 60 to cause the air inside the rotorholder 20 to travel toward the high temperature chamber wall surface505.

In the motor 100 according to the present preferred embodiment, theouter circumferential ends of the blades 60 are located radially outwardof the outer circumferential surface of the cylindrical portion 22 ofthe rotor holder 20. Adoption of the above arrangement makes it possibleto increase the velocity of the air current traveling radially outwardto efficiently cool the interior of the motor 100. As the blades 60 arelocated more radially outward, the circumferential velocity of eachrotating blade 60 increases. As the circumferential velocity of eachrotating blade 60 increases, a centrifugal force acting on the airaround the blades 60, the air rotating together with the blades 60, alsoincreases, and the flow velocity of the air current traveling radiallyoutward also increases. Therefore, when the outer circumferential endsof the blades 60 are located radially outward of the outercircumferential surface of the rotor holder 20, the interior of themotor 100 is efficiently cooled.

In the motor 100 according to the present preferred embodiment, eachblade 60 is preferably defined by a flat plate located in parallel orsubstantially in parallel with the rotation axis J, and extends in aradial direction. Adoption of the above arrangement makes it possible togenerate the air current traveling radially outward by employing thecentrifugal force. In addition, the air current traveling radiallyoutward is generated regardless of whether the shaft 1 is caused torotate in a normal direction or in a reverse direction.

In the motor 100 according to the present preferred embodiment, therotor vent holes 24 are located radially inward of the rotor magnet 21.Adoption of the above arrangement makes it possible to smooth a flow ofair which travels in the axial direction inside the rotor holder 20toward the rotor vent holes 24.

The rotor magnet 21 is fixed to the inner circumferential surface of thecylindrical portion 22 of the rotor holder 20. Accordingly, the flow ofair which travels in the axial direction inside the rotor holder 20 isgenerated radially inside the inner circumferential surface of the rotormagnet 21. Therefore, when the rotor vent holes 24 are located radiallyinward of the rotor magnet 21, a channel for the air inside the rotorholder 20 becomes straight or substantially straight, enabling a smoothflow of the air.

In the motor 100 according to the present preferred embodiment, thestator 3 includes the annular core back 30 b and the two or more teeth30 t extending radially outward from the core back 30 b. The rotor ventholes 24 are located radially outward of the core back 30 b. Adoption ofthe above arrangements makes it possible to smooth the flow of air whichtravels in the axial direction inside the rotor holder 20 toward therotor vent holes 24.

The core back 30 b is fixed to an outer circumferential surface of thebearing holding portion 11. Accordingly, the axial flow of air insidethe rotor holder 20 is generated radially outside the outercircumferential surface of the core back 30 b, that is, radially outsideradially inner ends of the teeth 30 t. Therefore, when the rotor ventholes 24 are located radially outward of the core back 30 b, the channelfor the air inside the rotor holder 20 becomes straight or substantiallystraight, enabling the smooth flow of the air.

In the motor 100 according to the present preferred embodiment, the twoor more rotor vent holes 24 are located in the circumferentialdirection. Adoption of the above arrangement makes it possible to smootha flow of air which passes in the axial direction through the bottomplate portion 23 of the rotor holder 20, enabling a smooth flow of airfrom the rotor holder 20 to the impeller 6.

The motor 100 according to the present preferred embodiment includes theimpeller 6 including the rotating plate 61 and the blades 60. Therotating plate 61 is located axially below the rotor holder 20, is in orsubstantially in the shape of a flat plate and perpendicular orsubstantially perpendicular to the rotation axis J, and is configured torotate together with the shaft 1. Each of the blades 60 axially projectsfrom the rotating plate 61. Adoption of the above arrangements makes itpossible to block the radiant heat from the high temperature chamberwall surface 505 with the rotating plate 61. This contributes topreventing the radiant heat from increasing the temperature of the rotorholder 20.

In the motor 100 according to the present preferred embodiment, therotating plate 61 is spaced away from the bottom plate portion 23 of therotor holder 20. Adoption of the above arrangement defines a gap betweenthe rotating plate 61 and the rotor holder 20, and makes it difficultfor heat in the rotating plate 61, which blocks the radiant heat, to betransferred to the rotor holder 20. In addition, a flow of air whichpasses through each rotor vent hole 24 is made smooth.

In the motor 100 according to the present preferred embodiment, therotating plate 61 is preferably defined by a metal sheet subjected to,for example, a stamping process, and the blades 60 are defined by, forexample, subjecting the rotating plate 61 to the lancing process.Adoption of the above arrangements makes it possible to easily producethe impeller 6 from the single metal sheet.

In the motor 100 according to the present preferred embodiment, the ribs62 are defined between the adjacent blades 60 in the rotating plate 61.Adoption of the above arrangement improves the strength of the rotatingplate 61. In addition, the ribs 62 contribute to generating the aircurrent traveling radially outward.

In the motor 100 according to the present preferred embodiment, eachblade 60 projects axially downward from the rotating plate 61. Adoptionof the above arrangement makes it possible to generate the air currenttraveling radially outward on a side of the rotating plate 61 closer tothe high temperature chamber wall surface 505. This contributes topreventing the high-temperature air in the vicinity of the hightemperature chamber wall surface 505 from approaching the rotor holder20.

In the motor 100 according to the present preferred embodiment, aportion of each opening 63 is located radially inward of an innercircumferential end of a corresponding one of the blades 60 in therotating plate 61. Adoption of the above arrangement makes it possibleto cause the air inside the rotor holder 20 to travel toward the hightemperature chamber wall surface 505 by employing the negative pressureproduced by the rotation of each blade 60. In particular, when an innercircumferential end of each opening 63 is located radially inward of theinner circumferential end of a corresponding one of the blades 60, alarger area of the opening 63 is secured radially inside the outercircumferential end of the blade 60. Accordingly, the flow of air whichpasses through the rotating plate 61 is increased, and the interior ofthe motor 100 is effectively cooled.

In the case where a portion of the rotating plate 61 is cut and erectedin the circumferential direction to define each blade 60, for example, acut hole is defined circumferentially adjacent to the blade 60. The cuthole is further expanded radially inward and is used as the opening 63.Thus, a large area for the opening 63 is secured.

In the motor 100 according to the present preferred embodiment, thelower end of each blade 60 is arranged at an axial level lower than anaxial level of each outlet edge portion 56 of the housing 5. That is, atleast a portion of the outer circumferential end of each blade 60 willbe opposed to each main air outlet 55. This enables the air to smoothlytravel radially outward along the high temperature chamber wall surface505, and makes it possible to effectively cool the motor 100.

In the motor 100 according to the present preferred embodiment, an upperend of each blade 60 is located at an axial level lower than an axiallevel of each outlet edge portion 56 of the housing 5. That is, theentire outer circumferential end of each blade 60 will be opposed toeach main air outlet 55. This enables the air to smoothly travelradially outward along the high temperature chamber wall surface 505,and makes it possible to effectively cool the motor 100.

In the motor 100 according to the present preferred embodiment, theaxial distance between the lower end of each blade 60 and the lower endof each attachment portion 52 is shorter than the axial distance betweenthe rotor holder 20 and the rotating plate 61. Adoption of the abovearrangement shortens the distance between each blade 60 and the hightemperature chamber wall surface 505, and makes it possible toeffectively discharge the high-temperature air in the vicinity of thehigh temperature chamber wall surface 505 through the air currenttraveling radially outward.

In the motor 100 according to the present preferred embodiment, a halfor more of the area of each rotor vent hole 24 is covered with therotating plate 61 when viewed from below in the axial direction. Thatis, the area of a region where each opening 63 and a corresponding oneof the rotor vent holes 24 overlap with each other when viewed frombelow in the axial direction is less than a half of the area of therotor vent hole 24. Adoption of the above arrangement contributes toeffectively blocking the radiant heat from the high temperature chamberwall surface 505. In particular, in the case where the radiant heat fromthe high temperature chamber wall surface 505 is to be blocked moreeffectively, each rotor vent hole 24 is preferably entirely covered withthe rotating plate 61.

The motor 100 according to the present preferred embodiment ispreferably attached to the high temperature chamber wall surface 505when in use. In the motor 100, the blades 60, which are configured torotate together with the shaft 1 to generate the air current travelingradially outward, are arranged on the side of the rotor 2 closer to thehigh temperature chamber wall surface 505. In addition, the housing 5includes the base portion 50 located axially above the rotor 2, the twoor more attachment portions 52 attached to the high temperature chamberwall surface 505, which is located axially below the impeller 6, and thecover portion 51 arranged to join the base portion 50 and the attachmentportions 52 to each other. Further, the base portion 50 includes the airinlets 54.

Adoption of the above arrangements makes it possible to effectively coolthe interior of the motor 100. Once the shaft 1 starts rotating, the airin the space between the rotor and the high temperature chamber wallsurface 505 is discharged through each main air outlet 55. Accordingly,the outside air is taken in through each of the air inlets 54 defined inthe base portion 50 of the housing 5, and the outside air taken intravels in the axial direction inside the cover portion 51 of thehousing 5 toward the high temperature chamber wall surface 505.Accordingly, the high-temperature air in the vicinity of the hightemperature chamber wall surface 505 is discharged radially along thehigh temperature chamber wall surface 505, and the outside air, whichhas a low temperature, is taken in through the base portion 50 on a sideopposite to the side where the high temperature chamber wall surface 505exists, such that a flow of air which passes axially downward inside therotor holder 20 is generated to cool the interior of the motor 100.

In the motor 100 according to the present preferred embodiment, thecircuit board 4 is arranged between the base portion 50 and the rotor 2,and the board vent hole 4 h is defined in the circuit board 4. Adoptionof the above arrangements secures a channel for air passing through thecircuit board 4. Accordingly, the outside air taken in through each airinlet 54 is able to travel in the axial direction inside the coverportion 51 of the housing 5 toward the high temperature chamber wallsurface 505.

In the motor 100 according to the present preferred embodiment, a gap isdefined between the circuit board 4 and the base portion 50. Thisarrangement enables the outside air taken in through each air inlet 54of the base portion 50 to smoothly travel to the board vent hole 4 h ofthe circuit board 4. This arrangement is particularly suitable in thecase where no air inlet 54 and the board vent hole 4 h have a sufficientoverlapping area when viewed in the axial direction.

In the motor 100 according to the present preferred embodiment, theboard vent hole 4 h is preferably arranged to at least partially overlapwith at least one of the air inlets 54 when viewed from above in theaxial direction. Adoption of the above arrangement enables the outsideair taken in through the air inlet 54 to smoothly travel into the boardvent hole 4 h.

In the motor 100 according to the present preferred embodiment, at leasta portion of the board vent hole 4 h is located radially inward of thecylindrical portion 22 of the rotor holder 20. Adoption of the abovearrangement enables outside air which has been taken into the housing 5through any air inlet 54 and which has passed through the board venthole 4 h to be introduced into the rotor holder 20. Thus, a flow of airwhich passes in the axial direction inside the rotor holder 20 isgenerated to effectively cool the interior of the motor 100.

In the motor 100 according to the present preferred embodiment, a gapbetween the circuit board 4 and the upper end of the cylindrical portion22 of the rotor holder 20 has a width smaller than the radial width ofthe rotor magnet 21. Adoption of the above arrangement reduces thelikelihood that air radially outside the rotor holder 20 will beintroduced into the rotor holder 20 through the gap between the circuitboard 4 and the upper end of the cylindrical portion 22 of the rotorholder 20.

The outside air is introduced into the rotor holder 20 through each airinlet 54 and the board vent hole 4 h. Therefore, the motor 100 iseffectively cooled by promoting the introduction of air from above inthe axial direction while limiting introduction of air from radiallyoutside the rotor holder 20. In particular, each air inlet 54 is able tointroduce outside air which is away from the high temperature chamberwall surface 505 and which has a relatively low temperature, and themotor 100 is effectively cooled by taking such outside air into therotor holder 20.

In the motor 100 according to the present preferred embodiment, thecover portion 51 of the housing 5 is located radially outside the rotorholder 20, and a gap is defined between the cover portion 51 and therotor holder 20. Adoption of the above arrangement makes it possible toshield the rotor holder 20 from outside air radially outside the coverportion 51, and makes it hard for the high-temperature air which hasbeen discharged through each main air outlet 55 to return to the rotorholder 20. Cooling performance thus is improved.

In the motor 100 according to the present preferred embodiment, theupper end of the cover portion 51 of the housing 5 is continuous withthe base portion 50 over an angular range of 180 or more degrees.Adoption of the above arrangement makes it hard for the high-temperatureair which has been discharged through each main air outlet 55 to returnto the rotor holder 20. The cooling performance thus is improved.

In the motor 100 according to the present preferred embodiment, thecover portion 51 of the housing 5 preferably includes the secondary airoutlet 57 defined at one circumferential position. Adoption of the abovearrangement enables air between the cylindrical portion 22 of the rotorholder 20 and the cover portion 51 of the housing 5 to be discharged outof the housing 5 through the secondary air outlet 57.

The rotation of the rotor 2 causes the air radially outside the rotorholder 20 to rotate in the circumferential direction. Therefore, whenthe secondary air outlet 57 is defined in the cover portion 51 of thehousing 5, the air radially outside the rotor holder 20 is dischargedout of the housing 5 through the centrifugal force. For example, ahigh-temperature air which has not been discharged through any main airoutlet 55 is discharged out of the housing 5 through the secondary airoutlet 57. This leads to effective cooling of the interior of the motor100.

In the motor 100 according to the present preferred embodiment, thesecondary air outlet 57 of the housing 5 is joined to one of the mainair outlets 55 of the housing 5. Adoption of the above arrangementcontributes to increasing the axial dimension of the secondary airoutlet 57 to more effectively discharge the air through the secondaryair outlet 57.

In the motor 100 according to the present preferred embodiment,preferably only one secondary air outlet 57, arranged at onecircumferential position, is defined in the housing 5. Adoption of theabove arrangement contributes to effectively cooling the interior of themotor 100. In the case where only one secondary air outlet 57, locatedat one circumferential position, is defined in the housing 5, dischargeof air is accomplished more easily than in the case where two or moresecondary air outlets 57 are defined in the housing 5. In addition, theflow velocity of the air which is discharged is increased. This enablesthe air, which has a high temperature, to be discharged farther away tomore effectively cool the motor 100.

In the motor 100 according to the present preferred embodiment, thesecondary air outlet 57 of the housing 5 is located radially oppositethe cylindrical portion 22 of the rotor holder 20. The discharge of theair through the secondary air outlet 57 is accomplished by using acentrifugal force generated as a result of air on an outer circumferenceof the rotor holder 20 rotating in the circumferential direction.Therefore, when the secondary air outlet 57 is located radially oppositethe cylindrical portion 22 of the rotor holder 20, the discharge of theair through the secondary air outlet 57 is effectively accomplished.

In the motor 100 according to the present preferred embodiment, theimpeller 6 has an outside diameter smaller than the inside diameter ofthe cover portion 51 of the housing 5. Adoption of the above arrangementcontributes to miniaturization of the motor 100.

In the motor 100 according to the present preferred embodiment, eachmain air outlet 55 has a circumferential dimension greater than thecircumferential dimension of each attachment portion 52 of the housing5. Adoption of the above arrangement causes each main air outlet 55 tohave a large circumferential dimension, and contributes to efficientdischarge of the air. In addition, a reduction in the circumferentialdimension of each attachment portion 52 leads to a reduction in an areaof contact between the attachment portion 52 and the high temperaturechamber wall surface 505, and to a reduction in heat which istransferred to the housing 5.

Second Preferred Embodiment

In the motor 100 according to the above-described preferred embodiment,the housing 5 preferably includes the secondary air outlet 57. Incontrast, in a motor 101 according to a second preferred embodiment ofthe present invention, a housing 5 preferably includes no secondary airoutlet 57. The motor 101 will now be described below.

FIG. 15 is a diagram illustrating an exemplary structure of the motor101 according to the second preferred embodiment of the presentinvention, and illustrates a section of the motor 101 taken along aplane including a rotation axis J. The motor 101 according to thepresent preferred embodiment is identical to the motor 100 according tothe first preferred embodiment except that no secondary air outlet 57 isprovided. Accordingly, like members or portions are designated by likereference numerals, and redundant description is omitted.

In the motor 101, a cover portion 51 of the housing 5 includes nosecondary air outlet 57. Therefore, of the air currents Fa1, Fa2, and Fbillustrated in FIG. 14, the air current Fb, which flows in thecircumferential direction, is not generated while the air currents Fa1and Fa2, each of which flows in the axial direction, are stillgenerated. More specifically, in a space radially between a rotor holder20 and the housing 5, air is caused to rotate in the circumferentialdirection because of rotation of a rotor 2, but the air is notdischarged through the secondary air outlet 57.

In the motor 101 according to the present preferred embodiment, nosecondary air outlet 57 is provided, and a cylindrical portion 22 of therotor holder 20 is opposed to the cover portion 51 of the housing 5throughout the entire circumference. That is, the cylindrical portion 22of the rotor holder 20 is covered with the cover portion 51 of thehousing 5.

Third Preferred Embodiment

In each of the motors 100 and 101 according to the above-describedpreferred embodiments, each blade 60 preferably projects axiallydownward from the rotating plate 61. In contrast, a motor 102 accordingto a third preferred embodiment of the present invention, each of blades60 preferably projects axially upward from a rotating plate 61. Themotor 102 will now be described below.

FIG. 16 is a diagram illustrating an exemplary structure of the motor102 according to the third preferred embodiment of the presentinvention, and illustrates a section of the motor 102 taken along aplane including a rotation axis J. The motor 102 according to thepresent preferred embodiment is identical to the motor 100 according tothe first preferred embodiment except in the shape of an impeller 6.Accordingly, like members or portions are designated by like referencenumerals, and redundant description is omitted.

In the impeller 6 of the motor 102, each of the blades projects axiallyupward from the rotating plate 61. Accordingly, once the impeller 6starts rotating, an air current traveling radially outward is generatedaxially above the rotating plate 61. Therefore, air which travelsaxially downward through any rotor vent hole 24 turns radially outwardwithout passing through any opening 63 of the rotating plate 61, and isdischarged through main air outlets 55. Therefore, according to thepresent preferred embodiment, an axial passage of air is smoother thanin the case of the motor 100, according to which air passes through eachopening 63 of the impeller 6 and is then discharged.

Fourth Preferred Embodiment

In each of the motors 100 to 102 according to the above-describedpreferred embodiments, the rotor holder 20 and the impeller 6 arelocated axially opposite each with a gap intervening therebetween. Incontrast, in a motor 103 according to a fourth preferred embodiment ofthe present invention, an impeller 6 is in contact with a bottom plateportion 23 of a rotor holder 20. The motor 103 will now be describedbelow.

FIG. 17 is a diagram illustrating an exemplary structure of the motor103 according to the fourth preferred embodiment of the presentinvention, and illustrates a section of the motor 103 taken along aplane including a rotation axis J. The motor 103 according to thepresent preferred embodiment is identical to the motor 100 according tothe first preferred embodiment except in the axial position of theimpeller 6. Accordingly, like members or portions are designated by likereference numerals, and redundant description is omitted.

The impeller 6 of the motor 103 is preferably fixed to a shaft 1 whilebeing in contact with the rotor holder 20. That is, no gap is definedbetween a rotating plate 61 of the impeller 6 and the bottom plateportion 23 of the rotor holder 20. Accordingly, the motor 103 is able tohave an axial dimension smaller than that of the motor 100, and toachieve a reduction in size. Note that, since no gap is defined betweenthe impeller 6 and the rotor holder 20, each of openings 63 of theimpeller 6 overlaps with at least one of rotor vent holes 24. Adoptionof the above arrangements makes it possible to effectively cool aninterior of the motor 103, and to reduce the axial dimension of themotor 103 to reduce the size of the motor 103.

Fifth Preferred Embodiment

In each of the motors 100 to 103 according to the above-describedpreferred embodiments, the impeller 6 preferably includes only onerotating plate 61. In contrast, in a motor 104 according to a fifthpreferred embodiment of the present invention, an impeller 6 preferablyincludes two rotating plates 61 a and 61 b. The motor 104 will now bedescribed below.

FIG. 18 is a diagram illustrating an exemplary structure of the motor104 according to the fifth preferred embodiment of the presentinvention, and illustrates a section of the motor 104 taken along aplane including a rotation axis J. The motor 104 according to thepresent preferred embodiment is identical to the motor 100 according tothe first preferred embodiment except in the shape of the impeller 6.Accordingly, like members or portions are designated by like referencenumerals, and redundant description is omitted.

The impeller 6 of the motor 104 preferably includes an upper rotatingplate 61 a arranged on a side closer to a rotor 2, a lower rotatingplate 61 b located on a side closer to the high temperature chamber wallsurface 505, and two or more blades 60 located between the upperrotating plate 61 a and the lower rotating plate 61 b.

The upper rotating plate 61 a and the blades 60 correspond to theimpeller 6 of the motor 100 according to the first preferred embodiment.For example, each blade 60 is preferably defined by, for example,subjecting the upper rotating plate 61 a to the lancing process, and anopening 63 is defined as a result.

Meanwhile, the lower rotating plate 61 b is used as a blocking plate.More specifically, the radiant heat from the high temperature chamberwall surface 505 is blocked by both the upper rotating plate 61 a andthe lower rotating plate 61 b. In the case where the lower rotatingplate 61 b also includes openings 63, the radiant heat is effectivelyblocked by arranging each opening 63 of the lower rotating plate 61 bnot to overlap with any opening 63 of the upper rotating plate 61 a whenviewed in the axial direction. Moreover, in the case where no openings63 are defined in the lower rotating plate 61 b, the radiant heat ismore effectively blocked.

In the motor 104 according to the present preferred embodiment, the twoor more blades 60 are located between the upper rotating plate 61 a andthe lower rotating plate 61 b to define the impeller 6. Adoption of theabove arrangement makes it possible to more effectively block theradiant heat from the high temperature chamber wall surface 505.

Sixth Preferred Embodiment

In each of the motors 100 to 104 according to the above-describedpreferred embodiments, the blades 60 are preferably disposed on therotating plate 61. In contrast, in a motor 105 according to a sixthpreferred embodiment of the present invention, blades 60 are preferablydisposed on a rotor holder 20. The motor 105 will now be describedbelow.

FIG. 19 is a diagram illustrating an exemplary structure of the motor105 according to the sixth preferred embodiment of the presentinvention, and illustrates a section of the motor 105 taken along aplane including a rotation axis J. The motor 105 according to thepresent preferred embodiment is identical to the motor 100 according tothe first preferred embodiment except that no impeller 6 is provided andthe blades are arranged on the rotor holder 20. Accordingly, likemembers or portions are designated by like reference numerals, andredundant description is omitted.

The motor 105 preferably does not include a rotating plate 61, and theblades 60 are instead attached to an outer circumferential surface of acylindrical portion 22 of the rotor holder 20. Accordingly, rotation ofa rotor 2 causes the blades to rotate to generate an air currenttraveling radially outward.

At a lower end of each blade 60, a portion of an outer circumferentialend of the blade 60 will be opposed to each of main air outlets 55. Thisenables the air current traveling radially outward to smoothly passthrough each main air outlet 55 to an outside of a housing 5. Inaddition, since the lower end of each blade 60 is arranged at an axiallevel lower than an axial level of a bottom plate portion 23 of therotor holder 20, the above air current is generated axially below therotor holder 20, and a negative pressure is produced below the bottomplate portion 23 of the rotor holder 20. This makes it possible togenerate an axial flow of air inside the rotor holder 20 through rotorvent holes 24.

In the motor 105 according to the present preferred embodiment, theblades 60 are provided on the outer circumferential surface of thecylindrical portion 22 of the rotor holder 20, and the lower end of eachblade 60 is provided at an axial level lower than an axial level of alower surface of the bottom plate portion 23 of the rotor holder 20.Adoption of the above arrangements makes it possible to effectively coolan interior of the motor 105, and to reduce the axial dimension of themotor 105 to reduce the size of the motor 105.

Note that, although it has been assumed that each of the motors 100 to105 according to the above-described preferred embodiments is used asthe driving source of the stirring fan 511 of the microwave oven 500,motors according to preferred embodiments of the present invention arenot limited to such an application. In other words, various preferredembodiments of the present invention is applicable to any and allapplications, including a variety of motors used under a hightemperature environment, and, in particular, to motors attached toattachment surfaces having a high temperature. Also note that such anattachment surface may not necessarily be flat, that a motor accordingto a preferred embodiment of the present invention may be attached tothe attachment surface in any manner, and that the attachment surfaceand the manner of attaching the motor to the attachment surfaceaccording to each of the above-described preferred embodiments have beendescribed merely for the purpose of illustration and are not essentialto the present invention.

Also note that, although the motor according to each of theabove-described preferred embodiments preferably is an outer-rotormotor, preferred embodiments of the present invention are alsoapplicable to an inner-rotor motor in which a stator is located radiallyoutside and opposite a rotor with a gap intervening therebetween.

Features of the above-described preferred embodiments and themodifications thereof may be combined appropriately as long as noconflict arises.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. A self-cooled motor comprising: two or moreattachment portions; a shaft extending supported by a bearing so as tobe rotatable about a rotation axis extending in a vertical direction; arotor configured to rotate together with the shaft; a stator locatedradially opposite to the rotor; a housing configured to support thebearing and the stator; and an impeller located axially below the rotor,and configured to rotate together with the shaft to generate an aircurrent traveling radially outward; wherein the housing includes: a baseportion located axially above the rotor; the two or more attachmentportions, each attachment portion being located radially outward of therotor; and a cylindrical or substantially cylindrical cover portionconfigured to join the base portion and the attachment portions to eachother; a lower end of each attachment portion is located at an axiallevel lower than an axial level of the impeller; the base portionincludes an air inlet; the cover portion includes an air outlet locatedbetween adjacent ones of the attachment portions, and configured toconnect a space radially inside the housing and a space radially outsidethe housing with each other; and the two or more attachment portions areconfigured to attach the self-cooled motor to a surface of a hightemperature chamber such that the shaft will extend into the hightemperature chamber.
 2. The self-cooled motor according to claim 1,further comprising a circuit board located between the base portion andthe rotor, wherein the circuit board includes an opening definedtherein.
 3. The self-cooled motor according to claim 2, wherein a gap isdefined between the circuit board and the base portion.
 4. Theself-cooled motor according to claim 3, wherein the opening of thecircuit board at least partially overlaps with the air inlet when viewedfrom above in an axial direction.
 5. The self-cooled motor according toclaim 1, wherein the rotor includes a rotor holder and a rotor magnet,the rotor holder including a cylindrical portion and a bottom plateportion, the rotor magnet being fixed to an inside of the cylindricalportion of the rotor holder; and the bottom plate portion of the rotorholder includes a rotor vent hole defined therein.
 6. The self-cooledmotor according to claim 4, wherein the rotor includes a rotor holderand a rotor magnet, the rotor holder including a cylindrical portion anda bottom plate portion, the rotor magnet being fixed to an inside of thecylindrical portion of the rotor holder; and the bottom plate portion ofthe rotor holder includes a rotor vent hole defined therein.
 7. Theself-cooled motor according to claim 5, further comprising a circuitboard located between the base portion and the rotor; wherein thecircuit board includes an opening defined therein; and at least aportion of the opening of the circuit board is located radially inwardof the cylindrical portion of the rotor holder.
 8. The self-cooled motoraccording to claim 7, wherein a gap is defined between the circuit boardand an upper end of the cylindrical portion of the rotor holder; and thegap has a width smaller than a radial width of the rotor magnet.
 9. Theself-cooled motor according to claim 5, wherein the cover portion of thehousing is located radially outside the rotor holder with a gap definedbetween the cover portion and the rotor holder.
 10. The self-cooledmotor according to claim 1, wherein an upper end of the cover portion ofthe housing is continuous with the base portion over an angular range ofabout 180 or more degrees.
 11. The self-cooled motor according to claim10, wherein the cover portion of the housing includes at least oneopening each of which is located at one circumferential position. 12.The self-cooled motor according to claim 11, wherein at least one of theat least one opening of the cover portion of the housing is defined as acut portion at a lower end of the cover portion of the housing.
 13. Theself-cooled motor according to claim 11, wherein the at least oneopening of the cover portion of the housing includes only one openinglocated at only one circumferential position.
 14. The self-cooled motoraccording to claim 12, wherein the at least one opening of the coverportion of the housing includes only one opening located at only onecircumferential position.
 15. The self-cooled motor according to claim11, wherein at least one of the at least one opening of the coverportion of the housing is located radially opposite to the cylindricalportion of the rotor holder.
 16. The self-cooled motor according toclaim 1, wherein the impeller has an outside diameter smaller than aninside diameter of the cover portion of the housing.
 17. The self-cooledmotor according to claim 5, wherein the impeller includes a blade; andan outer circumferential end of the blade is located radially outward ofan outer circumferential surface of the cylindrical portion of the rotorholder.
 18. The self-cooled motor according to claim 1, wherein theimpeller includes two or more blades; and each blade is defined by aflat plate parallel or substantially in parallel with the rotation axis,and extends in a radial direction.
 19. The self-cooled motor accordingto claim 1, wherein the air outlet has a circumferential dimensiongreater than a circumferential dimension of each attachment portion. 20.The self-cooled motor according to claim 1, wherein the housing isdefined by a press-worked material.